Table of contents

R. V. Khokhlov's (1926-1977) papers on nonlinear optics and nonlinear acoustics are reviewed. TABLE OF CONTENTS 1. Nonlinear waves and nonlinear oscillations 589 2. Method of step-by-step simplification of truncated equations: Khokhlov's method 590 3. Approximate equations of nonlinear optics 592    3.1. Plane waves: Space-time analogies. 3.2. Diffraction of light in a nonlinear medium. 4. Method of a slowly varying profile: Khokhlov's method in nonlinear acoustics 596 5. Random nonlinear waves 599 6. Theory of the γ-ray laser; coherent x-ray optics 601 7. Autowaves; pronounced nonlinearities 602 8. Conclusion 602 References 604

This review discusses nonlinear acoustic phenomena which are analogous to corresponding phenomena in optics: self-interaction effects in acoustic beams, parametric effects, stimulated scattering of sound, wave-front (phase) conjugation of sound waves, and also techniques of active acoustic spectroscopy. The authors point out that these processes must evolve rather rapidly in space and time in order to avoid the formation of shock waves and the nonlinear dissipation of energy which accompanies them; this rapid evolution is provided by the interaction of sound with non-acoustic forms of fluid motion: convection, oscillating bubbles, and by thermal, hydrodynamic and concentration waves. Features which are peculiar to nonlinear acoustic phenomena are emphasized, i.e., the parametric mechanism for acoustic phase conjugation and the possibility of detecting stimulated acoustic scattering by single scatterers. The goal of the review is to draw additional attention to promising developments in nonlinear acoustics.

Experiments in statistical nonlinear acoustics are reviewed. Measurements of the average intensities of the harmonics of a narrow-band randomly modulated signal, the nonlinear transformation of broad aerodynamic-noise spectra, and different effects involving the interaction of regular and random waves (active suppression of noise by an intense signal, excess fading of a weak signal in noise fields, cascade-like broadening of spectra, formation of white noise, and other effects) are described. Theoretical explanations are given for the observed phenomena. An approximate method is developed for finding the stochastic solutions of equations of the Burgers and Khokhlov-Zabolotskaya type. Results on the diffraction of intense noise, taking into account the effects of the spatial and temporal statistics, the excitation of random waves by distributed sources, and the formation of steady-state spectra, are presented for the first time. The problems of the nonlinear transformation of the statistical characteristics of acoustic noise and other general questions are discussed.

The review describes the present status of the physics of optical pulse self-action phenomenon. A brief analysis of linear propagation of ultrashort laser pulses in dispersive media and optical elements is given. Results on self-phase-modulation and on profile shock waves in media with various group velocity dispersion are presented. The problems of optical pulse compression and the possibilities to control their shape are discussed in detail. Particular attention is paid to the physical aspects of formation and interaction of optical solitons. Stochastic problems of temporal self-action are also discussed. In conclusion the promising directions in nonlinear optics of femtosecond laser pulses are discussed.

Studies on the electrodynamic theory of the process of readout of holograms recorded in the interference of an object speckle field and a smooth reference wave are reviewed. These holograms are classified as thin, thick-layer, and volume holograms, and the process of reconstitution is examined from a unitary standpoint for all these types of holograms. Calculations are performed of the diffraction efficiency and noise level with account taken of various physical factors: saturation of the photoresponse, shift in frequency and angle during reconstruction, shrinkage of the hologram material, etc. Substantial advantages of volume holograms are demonstrated with respect to efficiency and quality of reconstruction in the next five years and in the prospects up to the year 2000.

CONTENTS 1. Introduction 703 2. General requirements for laser media using solar excitation 704 3. Lasers with direct excitation by solar light 705 3.1. Basic characteristics of laser media. 3.2. Photodissociation Br₂–CO₂ lasers. 3.3. Interhalogen molecule lasers. 3.4. Iodine lasers. 3.5. Alkali metal vapor lasers. 4. Lasers with thermal conversion of solar pumping 709 4.1. General considerations. 4.2. CO₂ laser with excitation in a black body cavity and with gas flow. 4.3. cw CO₂ laser without gas flow. 5. Space laser media with solar excitation 713 5.1. Population inversion of molecular levels in the outer atmosphere of the Earth. 5.2. Laser effect in the atmospheres of Venus and Mars. 5.3. Terrestrial experimental technique for observing infrared emission in the atmospheres of planets. 5.4. Designs for laser systems in the atmospheres of Venus and Mars. 6. Conclusions 717 References 717